Read BLOCKHASH from storage and update cost

Abstract

Update the BLOCKHASH (0x40) opcode to read and serve from the system contract storage and charge the additional (cold or warm) storage costs.

Motivation

The BLOCKHASH (0x40) opcode currently assumes that the client has knowledge of the previous blocks, which in Verkle EIP-6800 would prevent stateless execution. However with EIP-2935 blockhashes can be retrieved and served from its system contract storage which allows Verkle blocks to include a storage access witness for stateless execution.

Specification

The BLOCKHASH opcode semantics remains the same as before. From the fork_block (defined as fork_block.timestamp >= FORK_TIMESTAMP and fork_block.parent.timestamp < FORK_TIMESTAMP), the BLOCKHASH instruction should be updated to resolve block hash in the following manner:

def resolve_blockhash(block: Block, state: State, arg: uint64):
  # note that outside the BLOCKHASH_SERVE_WINDOW we continue to return 0
  # despite the 2935 history contract being able to serve more hashes
  if arg >= block.number or (arg + BLOCKHASH_SERVE_WINDOW) < block.number
    return 0

  # performs an sload on arg % HISTORY_SERVE_WINDOW including gas charges,
  # warming effects as well as execution accesses
  #
  # note that the `BLOCKHASH_SERVE_WINDOW` and the 2935 ring buffer window
  # `HISTORY_SERVE_WINDOW` for slot calculation are different
  return state.load_slot(HISTORY_STORAGE_ADDRESS, arg % HISTORY_SERVE_WINDOW)

ONLY if the arg is within the correct BLOCKHASH window, clients can choose to either

• do a direct SLOAD from state, or
• do a system call to EIP-2935 contract via its get mechanism (caller other than SYSTEM_ADDRESS) or
• serve from memory or as per current designs if maintaining requisite history (full clients for e.g.)

However the entire semantics and after effects of the SLOAD operation needs to be applied as per the current fork if the arg is within the correct BLOCKHASH window:

• SLOAD gas costs (cold or warm) for the arg % HISTORY_SERVE_WINDOW slot.
• SLOAD after effects on the slot (warming the slot)
• SLOAD accesses added to execution witnesses if Verkle (EIP-6800 and EIP-4762) is activated

Activation

This EIP specifies the transition to the new logic assuming that EIP-2935 has been activated:

• sufficiently ahead of this EIP's activation (>= BLOCKHASH_SERVE_WINDOW) or
• at genesis for testnets/devnets where this EIP could also be activated at genesis

The current proposal is to activate this EIP with Verkle to allow for stateless execution of the block.

Gas costs

As described above, if the arg to be resolved is within the correct window, the corresponding SLOAD charges and accesses are to be applied for the slot arg % HISTORY_SERVE_WINDOW. Note that the HISTORY_SERVE_WINDOW and BLOCKHASH_SERVE_WINDOW are different.

Reading from the System contract

Even if the clients choose to resolve BLOCKHASH through system call to EIP-2935 contract, the gas cost for the system code execution (and also the code witnesses if Verkle activated) is not applied. Only the effect of SLOAD is applied as described above.

Rationale

• The reason behind the updated gas cost is to match the real operation, which is equivalent to an SLOAD.
• The EIP-2935 system contract execution charges (and accesses) are not applied to keep the gas low and to keep things simple for clients which choose to resolve BLOCKHASH in other ways (directly or though memory/maintained history)

Note that BLOCKHASH opcode only serves a limited BLOCKHASH_SERVE_WINDOW to be backward compatible (and to not extend the above exemptions). For deeper accesses one will need to directly call EIP-2935 system contract which will lead to a normal contract execution (as well as charges and accesses)

Backwards Compatibility

This EIP introduces a significant increase in the cost of BLOCKHASH, which could break use-cases that rely on the previous gas cost. Also, this EIP introduces a breaking change in the case where less than BLOCKHASH_SERVE_WINDOW elapse between the EIP-2935 fork and this EIP's fork (unless EIP-2935 is activated in genesis for e.g. in testnets/devnets) as the EIP-2935 system contract would not have saved the required history.

Test Cases

• If BLOCKHASH is not called or there is no EIP-2935 contract call by any transaction, only the EIP-2935 system update of the parent hash shows up in witnesses if Verkle is activated.
• If BLOCKHASH is called, there MUST be a storage access gas charge (and corresponding access witness if Verkle is activated) for the storage slot but ONLY if the BLOCKHASH query is for the last BLOCKHASH_SERVE_WINDOW ancestors. This is irrespective of how the client chooses to resolve the BLOCKHASH (directly, via system contract or via memory)
• The gas cost for each BLOCKHASH operation should still be charged, in addition to the SLOAD cost of each lookup (if performed)
• If the EIP-2935 contract is called directly (i.e. not through BLOCKHASH), then the witness and gas costs (including those related to contract code) are applied as per normal contract execution of the current fork.
• BLOCKHASH should be consistently resolved if this EIP is activated correctly >= BLOCKHASH_SERVE_WINDOW after EIP-2935

Security Considerations

No security considerations other than the ones contained in EIP-2935 are determined as of now.

Copyright

Copyright and related rights waived via CC0.